Presently, as a next-generation communication standard of a LTE (Long Term Evolution) system, specifications to design a LTE-Advanced system and its enhancement are being developed. In the LTE-Advanced system, a carrier aggregation (CA) technique is introduced to achieve a higher throughput than a LTE system while ensuring backward compatibility with the LTE system. In the carrier aggregation, a component carrier (CC) having the maximum bandwidth of 20 MHz supported by the LTE system is utilized as a basic component, and it is designed to implement communications in a wider band by employing such multiple component carriers simultaneously.
In the carrier aggregation, a user equipment (UE) can communicate with a base station (evolved NodeB: eNB) by using multiple component carriers simultaneously. In the carrier aggregation, a primary cell (PCell) having a high reliability to ensure connectivity to the user equipment and a secondary cell (SCell) additionally configured for the user equipment connecting to the primary cell are configured. The primary cell is similar to a serving cell in the LIE system and serves as a cell to ensure connectivity between the user equipment and a network. On the other hand, the secondary cell is a cell configured for the user equipment in addition to the primary cell.
In inter-band carrier aggregation where different frequency bands are used simultaneously, there is a case where harmonic effect may arise due to a relative relationship between combinations of frequency bands and frequency positions owned by an operator in the frequency bands. Specifically, a harmonic of a transmission band at a lower frequency band side falls in a reception band at a higher frequency band side, which may degrade sensitivity in the reception band.
In the current LTE specification, it is known that for a combination of frequency band 4 (B4) and frequency band 12 (B12), a harmonic may degrade reception sensitivity. In the LTE specification, it is defined that the lower frequency band 12 consists of an uplink frequency band (B12 UL) of 699-716 MHz and a downlink frequency band (B12 DL) of 729-746 MHz, and the higher frequency band 4 consists of an uplink frequency band (B4 UL) of 1710-1755 MHz and a downlink frequency band (B4 DL) of 2110-2155 MHz. For the combination of the frequency band 4 and the frequency band 12, as illustrated in FIG. 1, it is known that the third harmonic arising from transmissions of 699-709 MHz in the B12 UL falls in the reception band of 2110-2127 MHz in the B4 DL, which may degrade the reception sensitivity.
Also, it is known that for a combination of frequency band 3 (B3) and frequency band 8 (B8), a harmonic may also degrade the reception sensitivity. In the LTE specification, it is defined that the lower frequency band 8 consists of an uplink frequency band (B8 UL) of 880-915 MHz and a downlink frequency band (B8 DL) of 925-960 MHz and the higher frequency band 3 consists of an uplink frequency band (B3 UL) of 1710-1785 MHz and a downlink frequency band (B3 DL) of 1805-1880 MHz. For the combination of the frequency band 3 and the frequency band 8, as illustrated in FIG. 2, the second harmonic arising from transmissions at 905-915 MHz of the B8 UL falls in a reception band of 1810-1830 MHz of the B3 DL, which may degrade the reception sensitivity.
Presently, the specification is defined based on two patterns with respect to degradation of the reception sensitivity due to the harmonics. In the first pattern, a low-pass filter is inserted in a user equipment to provide support for the harmonic. The introduction of the low-pass filter causes the harmonic to be cut, which improves the degradation of the reception sensitivity in the higher frequency band side. On the other hand, transmission and reception characteristics must be relaxed in compensation. For example, for the combination of the frequency band 4 and the frequency band 12 in the carrier aggregation as stated above, it is assumed that a low-pass filter is inserted between a duplexer and a switch for the lower frequency band 12 so as to cut the harmonic. In the LTE specification, it is designed to relax the transmission and reception characteristics based on this assumption. Specifically, as illustrated in FIG. 3, a tolerance of a lower limit of maximum transmission power and reception sensitivity for the frequency band 12 are relaxed. As illustrated, for the combination of the frequency band 4 and the frequency band 12 (CA_4A-12A), the tolerance ΔTIB,c of the lower limit of the maximum transmission power is set to 0.8 dB, and 0.5 dB corresponding to a loss caused by the low-pass filter is added compared to the other tolerances 0.3 dB. Also, for the combination of the frequency band 4 and the frequency band 12 (CA_4A-12A), the reception sensitivity of the frequency band 4 is set to −90 dBm, that is, the reception sensitivity of the frequency band 4 is relaxed by 10 dBm from −100 dBm for the case where no low-pass filter is installed. Correspondingly, the communication area is reduced. The first pattern is applied to the case where sensitivity degradation due to the harmonic may arise in operator's actually providing services.
In the second pattern, on the other hand, no support for the harmonic is implemented, although the above-stated sensitivity degradation due to the harmonic arises. From a viewpoint of frequency positions owned by an operator, there is a case where the sensitivity degradation due to the harmonic is of no matter in operator's actually providing services. To this end, no support for the harmonic such as insertion of a low-pass filter is implemented, and the relaxation of the transmission and reception characteristics as stated above in conjunction with the first pattern is not defined. For example, for the combination of the frequency band 3 and the frequency band 8 as illustrated in FIG. 2, it is known that the second harmonic arising from transmissions at 905-915 MHz in the B8 UL may fall in the reception band of 1810-1830 MHz in the B3 DL. Since there is no operator that uses these two frequency bands, however, no relaxation of the transmission and reception characteristics is currently defined for the combination of the frequency band 3 and the frequency band 8 differently from the combination of the frequency band 4 and the frequency band 12 as described with reference to FIG. 3.
See 3GPP TS36.101 V12.4.0 (2014 June) for further details, for example.